There was probably an earlier date in the year when she became eligible. It would be too expensive to make day by day adjustments to the relevant file of names. Best not to stress about it at this difficult time.
Many of you know my MIL died last week and thank you for your messages. Today DH as executor had a letter asking where to send the Winter Fuel Allowance having already notified all agencies of her demise. The answer is apparently that her estate is owed the money as she was eligible for it. I don't understand this MIL has died and will not be needing her WFA. Mad and wasteful.